The invention concerns the late, so-called xe2x80x9cin mouldxe2x80x9d, treatment of liquid cast irons intended for the manufacture of parts for which it is desired to obtain a structure free from iron carbides.
The treatment concerned is mainly inoculation treatment.
xe2x80x9cIn mouldxe2x80x9d treatment consists in placing the cast iron treatment product in the liquid cast iron casting system.
Cast iron is a well known iron-carbon-silicon alloy widely used for the manufacture of mechanical parts. It is known that in order to procure good mechanical properties for these parts, it is necessary in the end to obtain an iron+graphite structure while preventing as far as possible the formation of Fe3C type iron carbides which embrittle the alloy.
Thus it may be preferred for the formed graphite to be spheroidal, if a spheroidal graphite cast iron called xe2x80x9cSG ironxe2x80x9d or xe2x80x9cductile ironxe2x80x9d is required, rather than lamellar. If a lamellar graphite cast iron called xe2x80x9cLG ironxe2x80x9d or xe2x80x9cGrey ironxe2x80x9d is required, but the essential prior condition to be met is to prevent the formation of iron carbide.
To this end the liquid cast iron is subject before casting to an inoculation treatment, which will, as it cools, favour the appearance of graphite rather than that of iron carbide.
The inoculation treatment is therefore very important. It is in fact well known that inoculation, whatever the inoculants used, has on the liquid cast iron an effectiveness which reduces with time and which, generally, has already reduced by 50% after a few minutes. To obtain maximum effectiveness, the man skilled in the art generally practises progressive inoculation, applying to this end several additions of inoculants at different stages of the development of the cast iron; the final addition is made xe2x80x9cin mouldxe2x80x9d as the moulds are fed or even in the feed conduits of the moulds by placing in the path of the liquid cast iron inserts constituted by an inoculant material. These inserts are generally used associated with a filter; in this case they generally have a perfectly defined shape in order to be able to be fixed in the filter, most often in an adapted cavity; these inserts of defined shape are known as pellets. We will denote by the name xe2x80x9cinoculant filterxe2x80x9d the unit constituted by the slug and the filter.
There are two types of pellets:
xe2x80x9cmouldedxe2x80x9d pellets obtained by moulding the molten inoculant.
agglomerated pellets obtained from a pressed powder with generally very little binding agent, or even without binding agent.
Moulded pellets are considered, by the man skilled in the art, as being the best quality; however agglomerated pellets are often preferred to them for reasons of cost.
The object of the invention is a pellet, intended for the late inoculation of cast irons, obtained by agglomeration of a powdered inoculant, characterised in that the mass proportion of the granulometric fraction 50-250 microns of the powdered inoculant of which the pellet is constituted is comprised between 35 and 60%, and preferably between 40 and 50%, and the mass proportion of the granulometric fraction below 50 microns is lower than 25%, and preferably 20%. The particle size of the powder is preferably lower than 1 mm.
The man skilled in the art who practises inoculation at the different stages of the development of the cast iron uses products which are all the finer the later the inoculant is added in the process; the logic is that upstream the products have all the time necessary to dissolve and that when they reach the inlet of the moulds they have only a few seconds left before, solidification.
In this way, the granulometry bracket 2/10 mm is currently used in pre-inoculation, 0.2/2 mm during ladle treatment, and 0.2/0.7 mm for runner inoculation when casting the ladles. The applicant has in fact noted in the testing shop an unexpected phenomenon:
For a same dosing of inoculant, the number of graphite nuclei generated in the liquid cast iron increases with the number of inoculant particles added to the inoculant mass unit.
Therefore if two ladles of cast iron are treated in identical conditions with a same inoculant in two different particle size distributions, the cast iron treated with the finest product will contain more graphite nuclei than that treated with the coarser product; these nuclei will also be smaller in size.
The same phenomenon has been observed during an xe2x80x9cin mouldxe2x80x9d treatment with agglomerated slugs; the cast iron treated with a slug obtained from a finer powder will contain more graphite nuclei than that treated with a pellet obtained from a coarser powder; these nuclei will also be smaller in size.
This fairly unexpected observation may have advantageous applications since it may make it possible to control the density of the graphite nuclei in the cast iron part and therefore the structure of the manufactured part.
To obtain pellets in this way which have maximum effectiveness in terms of inoculation, the applicant has been led to prepare powders at 0/1 mm having a particular internal particle size distribution defined in the following way:
Passing to 1 mm: 100%.
Fraction between 50xcexcand 250xcexc: 30% to 60%, and preferentially 40% to 50%.
Fraction below 50xcexc: less than 25% and preferentially less than 20%.
A powder of this type agglomerates easily which makes it possible to operate with lower proportions of binding agent. Thus with sodium silicate which is a well-known binding agent, doses of 0.3 cm3 for 100 g of powder to 3 cm3 for 100 g of powder are sufficient according to the pressures employed which may vary from 50 to 500 Mpa; since the mechanical performance of the pellets is easily acquired, the pressure and binding agent percentage parameters may be used to control the dissolution speed of the pellet and not its mechanical performance.
However experience shows that the particle size distribution defined above cannot be obtained by natural crushing; the preparation of powder with this particle size distribution requires a dosing of size fractions prepared in isolation.
The inoculant composition can be obtained either by mixing powders of different elemental products, or in form of an alloy, powder, or by mixing powders of different alloys.